by Ken Task.
What does 'no theme' look like? Screen snap would be good. If blank screen (ie, white screen of death) then say that.
What is ownership/permissions on theme folder/files ... for all files/folders in the code directory for that matter.
Did the site fully qualified domain name change? or is it the same? *If it changed*, did you run search and replace tool to correct now errant full url's.
Is site running opcache or apc? Have you cleared those?
And, don't forget browser cache as well. ;)
'spirit of sharing', Ken